Computerworld - After a one-month pause, Microsoft’s Internet Explorer (IE) resumed its usage share slide in December, dropping to a new low and setting the stage for a fall below 50% as early as March.

IE lost eight-tenths of a percentage point last month to end with a share of 51.9%, according to California-based metrics company Net Applications. IE dropped more than seven points during 2011.

In November, said Net Applications, IE held steady, the only month in the year when it did not lose share.

Google’s Chrome benefited most from IE’s decline, growing its share by nine-tenths of a percentage point to a record high of 19.1%. Chrome should crack the 20% mark either this month or in February.

As was its practice during much of 2011, Microsoft did not address the continued slide of IE, but instead pointed to IE9′s performance on Windows 7, a combination the company has repeatedly said is the only metric that matters.

“Based on where the December data currently stands,” said Roger Capriotti, the head of IE marketing, in a Dec. 30 blog, “we’re pleased to say IE9 … will soon take the top spot from IE8 on Windows 7, with usage share expected to come in at nearly 25.6% this month.”

Neither Microsoft nor Net Applications — which does not make OS-specific browser statistics available to the public — published definitive December IE9-Windows 7 numbers on Jan. 1.

On all operating systems, IE9 held an 11.5% share during December, an increase of 1.2 points over November. IE8 remained Microsoft’s most-popular browser by far, accounting for 27.3% of all browsers used, or more than half of all versions of IE.

Other editions also slid last month: IE7 dropped to 4.8% and IE6, which Microsoft has been trying to euthanize since 2009, fell to 7.3%.

Microsoft’s recent decision to automatically upgrade older editions of IE may depress IE6 and IE7 numbers further during 2012 in favor of IE8 and IE9, but it’s unclear if the move will stem IE’s overall defections.

Chrome has gained most of what IE and Mozilla’s Firefox lost this year, boosting its numbers by 8.8 points in 2011. Firefox, which dropped three-tenths of a point during December and 1.9 points for the year, ended 2011 with 21.8%, a new low.

Firefox will lose its second-place spot to Chrome in March if the two browsers keep to their current Net Applications’ trend lines. (According to rival measurement company StatCounter, Chrome has already overtaken Firefox.)

Also during December, Apple’s Safari stayed flat at 5%, and Opera Software’s Opera gained one-tenth of a percentage point, its first significant increase in over a year, to reach 1.7%.

Net Applications calculates browser usage share with data obtained from more than 160 million unique visitors who browse 40,000 Web sites that the company monitors for clients. More browser statistics can be found on the company’s site.